Apparatus for burning sulfur containing fuels

ABSTRACT

Combustion apparatus for the removal of sulfur oxides from fuels containing sulfur. A first and a second fluidized bed boiler having heat exchange surfaces, that burn the sulfur fuels are positioned adjacent each other with the second bed having less heat exchange surface than the first. Each bed has solid particulate sulfur acceptors that are circulated between the two beds. Heat exchange wall members between the beds semi-isolate them from each other such that there is little or no mixing of the gaseous combustion products from each boiler. The circulation between beds may be accomplished by ports in the exchange wall below the top of the fluidized beds. The end result is the gas emitted to the atmosphere from the beds after additional processing is depleted in sulfur oxides.

United States Patent [191.

Robinson et a].

[4 Oct. 9, 1973 APPARATUS FOR BURNING SULFUR CONTAINING FUELS PrimaryExaminer-Kenneth W. Sprague [75] Inventors: Ernest B. Robinson, SilverSpring; A'mmey Frank Lukaslk et Shelton Ehrlich, Bowie, both of Md.;John W. Bishop, Alexandria, 57 ABSTRACT Va. [73] Assignec. The Unitedstates of America as Combustion apparatus for the removal of sulfuroxides represented by the secreary ofthe from fuels containingsulfur. Afirst and a second flu- Interior rdized bed boiler having heat exchangesurfaces, that burn the sulfur fuels are positioned adjacent each [22]Fled: 1973 other with the second bed having less heat exchange 2 APPL 55 surface than the first. Each bed has solid particulate sulfuracceptors that are circulated between the two Related Applicallon Danbeds. Heat exchange wall members between the beds [62] Division of Ser.No. 66,724, Aug. 25, 1970. semi-isolate them from each other such thatthere is little or no mixing of the gaseous combustion products [52]U.S. Cl. 122/4 D, 110/28 J from each boiler. The circulation betweenbeds may [51] Int. Cl. F22b l/02 be accomplished by ports in theexchange wall below [58] Field of Search 122/4 D; 1 10/28 J the top ofthe fluidized beds. The end result is the gas emitted to the atmospherefrom the beds after addi- [56] References Cited tional processing isdepleted in sulfur oxides.

UNITED STATES PATENTS 5 Claims, 4 Drawing Figures 2,842,102 7/1958Blaskowski l22/4 3,387,590 6/1968 Bishop l22/4 3,495,654 2/1970Jacubowiez... 122/4 4 3,508,506 4/1970 Bishop 122/4 TO STACK 1 APPARATUSFOR BURNING SULFUR 1 CONTAINING FUELS This application is a division ofpatent application having U.S. Pat. Ser. No. 66,724 filed Aug. 25, I970,no.w'U.S. Pat. No. 3,717,700.

This invention arose under a project sponsored by the National AirPollution Control Administration of the U. S. Department of Health,Education, and Welfare. The project was conducted under contract withthe Office of Coal Research of the U. S. Department of the Interiorthrough an interagency transfer of funds from the NationalAir PollutionControl Administration.

BACKGROUND OF THE INVENTION There is an increasing public andgovernmental interest in curbing sulfur oxide pollution of theatmosphere.

Many localities have now enacted legislation severely restricting sulfurdioxide omissions in the products of combustion or the sulfur content offuel burned. These restrictions are having a major disruptive effectupon traditional combustion methods and fuel sources.

Much of the sulfur dioxide emitted to the atmosphere is produced by theburning of coal, particularly in power generation. Some authoritiesestimate that as high as 60 percent of the total sulfur dioxidepollution is caused by burning of coal. Historically in the UnitedStates, removal of sulfur dioxide has been practiced only with smeltergases where concentrations normally range from about 3 to 7 percent.Removal of sulfur oxides from power plant stack gases is much moredifficult because of the vastly greater gas flows and smaller sulfuroxide concentrations. Concentration of sulfur oxides in power plantstack gases generally ranges from about 0.2 to 0.5 percent; a fuelcontaining percent sulfur, for example, when burned in a conventionalboiler produces a flue gas containing about 0.5 percent sulfur oxides.

1 Removal of sulfur oxides from a gas stream is an old art. An excellentcompendium of this art will be found in theU. S. Bureau of MinesInformation Circular 7836 (1958). Since that time, major researchemphasis has been placed by the National Air Pollution ControlAdministration upon the development of improved absorbents for sulfuroxide removal. A description of the current stateof-the-art may beobtained from NAPCA publication AP-52, Control Techniques for SulfurOxide Air Pollutants.

Some prior art methods make use of limestone-based materials. In oneprocess, finely divided limestone is injected into a boiler furnace at apoint somewhat removed from' the flame where the limestone calcines andpartially reacts with sulfur oxides. The partially reacted solid is thenremoved from the flue gas by dry methods and is either wasted or sent toa regenerator.

In another process, finely divided limestone is again injected into aboiler furnace but the partially reacted solid is passed to a gasscrubber where it reacts with sulfur oxides as they pass into solution.A variation of this process passes the limestone product limestonederived material directly into the gas scrubber without first havingbeen injected into the furnace.

A third method uses particulate limestone as a fixed, moving orfluidized bed at temperatures in the range of about l,200 to about l,800F to contact and absorb sulfur oxides in a flue gas stream. Theresulting product may either be wasted or regenerated to drive off andrecover sulfur oxides.

Finally, it has been proposed to burn sulfur-bearing coal or oil in afluidized bed of limestone. Combustion in a fluidized bed may be carriedout at relatively low temperatures so that the calcined limestoneretains its activity toward sulfur oxides. If fuel is burned within sucha fluidized limestone bed under reducing conditions, then sulfur oxidesare fixed as the sulfide; if burned under generally oxidizing conditionsthey are fixed as the sulfate. It has been proposed to roast sulfideswith air in a separate regenerator to produce sulfur dioxide and toobtain calcium oxide for reuse in the system. It has also been proposedto react calcium sulfates under reducing conditions to obtain the sameproducts.

Specific processes taught in the prior art for regenerating sulfatedcalcium oxides include reduction by partial combustion of carbonmonoxide at about l,850 F; reduction by partial combustion of methane atapproximately 2,000 F; and reduction by partial combustion of fuel oilat about l,850 F. None of these processes for decomposing sulfatedcalcium oxides can be carried out in a boiler furnace since the reducinggases carburize boiler tubes thus weakening them and causing eventualfailure. From a practical viewpoint, use of reducing conditions requiresa separate regenerator.

SUMMARY OF THE INVENTION We have found that sulfur oxide acceptors, suchas calcined limestone, dolomite and the like, can be regenerated orforced to release sulfur dioxide within a fluidized bed combustionfurnace operating in an oxidizing mode. Thus, regeneration of a sulfuroxide acceptor can be carried out simultaneously with combustion andsteam generation.

In our most preferred embodiment, coal is burned within a plurality offluidized bed combustion zones in contact with'sulfur oxide acceptor.Noncombustible particulate material comprising the sulfur oxide acceptorfreely circulates between these combustion zones and a regeneration zonewhile flue gas issuing from the combustion zones is segregated from thatproduced in the regeneration zone. The regeneration zone is operated ata higher temperature than are the combustion zones. Sulfur oxidesabsorbed in the combustion zones is released as sulfur dioxide in thehigher temperature regeneration zone. As a result, flue gas issuing fromthe combustion zones is substantially depleted in sulfur oxides whileflue gas exhausting from the regeneration zone is substantially enrichedin sulfur dioxide.

Hence it is an object of our invention to provide apparatus to reduceatmospheric omissions of sulfur oxides from combustion ofsulfur-containing fuels.

It is another object of our invention to concentrate a major portion ofsulfur oxides produced in a boiler furnace within a small portion offlue gas.

Another object of our invention is to provide an apparatus forregenerating sulfur oxide acceptor materials.

DETAILED DESCRIPTION OF THE INVENTION When coal or other sulfurcontaining fuels are burned in a fluidized bed of limestone, threeidentifiable reactions can proceed simultaneously. First, combustion ofthe fueloccurs with releases of water vapor, carbon oxides and sulfuroxides depending upon fuel composition. Calcining of the limestone alsotakes place to produce a calcium oxide or lime which then can react withan absorb sulfur oxides.

There is a particular temperature range at which all three reactionsproceed at suitable rates and efficiencies. Burning of coal and similarfuels in a fluidized bed requires a minimum temperature of about l,400F. Below this temperature, combustion is unstable and tends to beincomplete. Complete combustion is defined for the purposes of thisdisclosure as the essentially complete oxidation of fuel gases, i.e.,carbon monoxide, hydrocarbons and hydrogen. It does not imply completecombustion of carbon particles leaving the combustor since these may beconveniently removed from the products of combustion and re-flred. Thatsame temperature, 1,400 F, has been found too low for rapid calcinationof carbonates such as limestone in a fluidized bed combustor. However,calcium carbonate calcines easily at temperatures of about l,500 F andhigher. Sulfur oxides are also readily accepted by calcined limestones,dolomites and the like at that same temperature, l,500 F, but theirreactivity towards the sulfur oxides declines with increasingtemperature reaching essentially zero above about 2,000 F. There is,however, a temperature range between about l,450 and l,750 F in whichall three reactions will proceed simultaneously at acceptable rates.Overall efficiency of the combined reactions increases toward the middleof that temperature range and preferred operating conditions includetemperatures between about ],500 to l,700 F with about 0.5 to percentunreacted oxygen appearing in the flue gas. Most preferred operatingconditions include a temperature range of about l,550 to 1,650 F and 23percent unreacted oxygen.

Reactions other than combustion which take place within the fluidizedbed are as follows:

I. Calcination of calcium carbonate.

caco, Ca0 co This reaction essentially goes to completion at nor maloperating conditions of the process.

Ca0 S 3/2 0, CaSO,

This second reaction is known to be far more complex than thegeneralized equation presented. For example, it is unknown exactly whatform the sulfur. is in at the time of reaction. Sulfldes and sulfitesmay be important reaction intermediates but only the sulfate has beenfound in analyzed samples taken from an operating bed. The reactionproceeds rapidly on the surface of calcined limestone particles and willquickly reach sulfur levels of 7.5 percent or more. A theoretical upperlimit of 24 percent sulfur corresponding to anhydrous calcium sulfate,may be approached in some operating conditions. i

In the prior art, sulfated calcium oxides, formed in whatever manner,are decomposed in a reducing environment as was previously described. Itis at this point that our invention diverges most strongly from theaccepted principles of the prior teachings. We decompose the sulfatedcalcium oxide in accordance with the following generalized reactions.

3. CaSO, CO Ca0 S0 CO CaSO, H Ca0 SO -l- H O Reduction and reducingconditions are indicated by the equations but the reaction is carriedout in an oxidizing fluidized bed. Temperature of the reaction must beabove about l,700 F and it is preferred to operate with small butmeasurable quantities of oxygen appearing in the flue gas. Much higherreaction temperatures, up to about 2,200 F, can be used. At the hightemperatures, much higher concentrations of oxygen can be tolerated inthe flue gas without interfering with the reaction. We prefer to carryout the decomposition or regeneration step at temperatures within therange of about l,700 to about 2, 1 00 F with a concentration ofunreacted oxygen in the flue gas of less than about 3 percent;specifically within the range of about 0.5 to about 2.5 percent.

Carrying out a reducing reaction within an environment which isgenerally oxidizing appears at first to be an anomaly. While we do notpretend to completely understand the details of the chemistry involved,we postulate that each burning fuel particle is surrounded by a halo ofhot reducing gas. We believe that the actual reaction takes place in avery localized manner within this gas halo before the gas is consumed bythe combustion process. At the temperatures utilized in thedecomposition or regeneration zone, little or no reaction between thereleased sulfur dioxide and the regenerated calcium oxide will occur.Hence, essentially complete decomposition of the sulfated calcium oxidecan easily be obtained.

Practical techniques and apparatus for utilizing our discovery will bemore thoroughly understood by reference to the accompanying drawings inwhich:

FIG. 1 comprises a generalized, diagrammatic flow sheet of ourinvention.

FIG. 2 is a cross-sectional view of a modular fluidized bed boileruseful in the practice of our invention.

FIG. 3 comprises a partial sectional view of preferred apparatus.

FIG. 4 shows construction details of the heat exchange walls separatingmodules of our combustion apparatus.

Referring now to FIG. 1, there is shown a generalized, diagrammatic flowsheet of a combustion process utilizing our invention to concentratesulfur oxides within a minor portion of the total flue gas stream. Thereis provided a first combustion zone 1 and a second combustion zone 2.Zone 1 may comprise a variety of conventional combustion devicesincluding pulverized coal or oil fired furnaces, fluidized bedcombustors and the like. Zone 2 comprises a fluidized bed boiler. Forpurposes of this disclosure, a fluidized bed boiler is a device whichmeets eachof the following three criteria:

1. Its primary function is to extract heat from a combustion process asby the generation of vapor such as steam.

2. Fuel is added to and is burned within a turbulent bed of particulatematerial maintained in the fluidized state.

i 3. A substantial fraction, from about 25 to about 60 percent of theheat released by the combustion is extracted by heat transfer surfacesin direct contact with the fluidized bed.

Devices of this sort are known as the Pope-Bishop boiler and a schematiccross sectional drawing of such a unit adapted for use in our processappears as FIG. 2.

Fuel, such as sulfur-containing coal, is introduced into zone 1 via feedmeans 3 where it is burned by combustion air entering via means 4. Asolid, particulate sulfur acceptor, such as limestone or dolomite isintroduced via line 5 into a portion of the combustion zone whereatcombustion products are at a temperature in the range of about l,200 toabout 2,500 F. In combustion devices such as pulverized coal furnaces,the sulfur acceptor is introduced in finely divided form at a pointsomewhat removed from the flame zone and is carried from the combustorin the exiting flue gas stream 6. During the time flue gas is inintimate contact with the sulfur acceptor, some of the sulfur oxidescontained within the flue gas are reacted with and absorbed by theacceptor material.

Exit gas stream 6 is then passed to a mechanical separation device 7which removes substantiallyall of the sulfated acceptor material alongwith the larger sized fraction of fly ash produced in the combustionprocess. This larger sized fraction of fly ash includes most of theunburned or partially burned carbon contained in the flue gas stream.There is recovered from separator'7 a flue gas stream 8 nowsubstantially depleted in entrained solids arid sulfur oxides. Stream 8may then be passed through a secondary solids removal device 9,

such as anelectrostatic precipitator, where nearly all of the remainingparticulate material is removed from the system via line 10. A cleanedflue gas stream 11 is then passed to the stack. Other arrangements ofparticulate removal devices may also be used.

Solid material recovered from separator 7 and comprising a mixture ofsulfated acceptor material, fly ash and unburned carbon is introducedvia conduit means 12 into an oxidizing fluidized bed maintained withincombustion zone 2. Also introduced into the fluidized bed issupplemental fuel supply 13 which preferably comprises particulate coal.Air stream 14 is provided to supply a slight excess of oxygen over thatrequired for complete combustion, as defined earlier, and to maintainthe particulate bed within zone 2 in a fluidized state.

Combustion zone 2 is maintained at a relatively high 1 ,closed in U. S.Pat. No. 3,508,506. More importantly,

however, zone 2 functions to decompose sulfated acceptor material torelease sulfur dioxide and regenerate the acceptor to an active form. Aproduct gas stream substantially enriched in sulfur oxides, is passed toseparating means 15 by way of conduit 16. Separator 15, like separator7, comprises a mechanical device such as a cyclone separator to removesolid particulate material from the gas stream. Solids recovered fromseparator l5 and comprising primarily regenerated sulfur acceptormaterial is passed via line 17 back to the first combustion zone.

A gas stream now containing most of the sulfur oxides produced in bothzones 1 and 2, is passed from separator 15 to sulfur oxide removal unit18 by way of conduit means 19. Unit 18 may comprise any conventionaltechnique for removing and preferably recovering elemental sulfur,sulfur oxides or sulfuric acid from the gas stream. Illustrativetechniques include wet scrubbing with water or other liquids such asglycols or amines and absorption of the sulfur oxides on solidabsorbents. In some cases when burning high sulfur fuels, unit 18 maycomprise a contact process sulfuric acid plant.

Concentration of sulfur oxides appearing in line 19 depends upon avariety of factors. Of primary importance is the sulfur content of fuelsburned in the two combustion zones. Secondly, the ratio of fuelquantities burned in the two zones directly affects the concentrationratio of sulfur oxides in flue gas stream 19 as compared to stream 8.Lately, the efficiency of the sulfur acceptor introduced into zone Idirectly affects the residual concentration of sulfur oxides appearingin the flue gas from the zone.

If a chemically active limestone is used as the sulfur acceptor in zoneI, and if stoichiometric ratios of CaO to S are maintained above about3, then more than percent of the sulfur oxides produced by combustion inthat zone can be absorbed by the acceptor and removed from the flue gasstream. Removals of 90 percent can be achieved if zone 1 is a fluidizedbed combustor. If zone 1 is a pulverized coal combustor removals may beon the order of 50 percent. Relative size of the two combustion zones ispreferably adjusted so that flue gas produced by zone 2 constitutes aminor fraction, preferably less than one-fourth, of the total flue gasstream from the two zones. Our work at this time indicates that sizingof the two combustion zones such that flue gas from zone 2 constitutesapproximately 10 percent of the total gas stream produces verysatisfactory and perhaps optimm overall process efficiencies. Thus,sulfur oxide content of stream 19 is enriched by a factor of about 10and sulfur oxide content of stream 8 is depleted by a factor of about 10as compared to an untreated flue gas stream. When burning coal having asulfur content of about 5 percent, sulfur oxide levels in the flue gasfrom zone 1 can be held below about 0.05 percent while the sulfur oxidecontent in the flue gas from zone 2 will be on the order of 4 to 6percent. In actual practice, we have achieved sulfur dioxide levels inexcess of 8 percent in the flue gas from a fluidized bed boiler used inthe manner described to decompose sulfated limestone acceptors.

When removing sulfur oxides from a waste gas by wet scrubbingtechniques, it is usually necessary to cool the gas substantially belowthe normal stack temperatures of 250 to 300 F. Conventionally, gases arecooled to a temperature level of perhaps 90 to F in order to maximizeefficiency of the scrubbing solution and to minimize solvent loss if thesulfur oxide solvent used is something other than water. When thescrubbed gas is then released to the atmosphere, it is usually saturatedin water vapor and produces a large and undesirable steam plume. Becauseof the loss in buoyancy of the scrubbed gas due to cooling, it tends toremain close to ground level and not disperse as does the hotter stackgas. In order to avoid these problems when using wet scrubbingtechniques for sulfur oxide removal, we prefer to merge the sulfuroxide-depleted gas stream 20 from removal unit 18 with the hotter majorflue gas stream 11 prior to entry into the stack. In this manner, littlebuoyancy loss of the merged flue gas streams is experienced and thecombined gas stream can be efficiently dispersed by the stack.

Up to this point, the process illustrated in FIG. 1 has described use ofour invention with a relatively finely divided particulate sulfuracceptor. It is also possible, and in some cases preferred, to use anacceptor having a much larger particle size. For example, if the deviceused to carry out the combustion in zone I is a fluidized bed combustoror similar device, then particulate sulfur acceptor material such aslimestone may be used as the fluidized bed. In this variation of ourprocess, little sulfated acceptor material is carried out of zone 1 inthe existing flue gas stream. Rather, circulating means 21 are providedto transport sulfated bed material from zone 1 to zone 2 and returnmeans 22 are provided to transport regenerated sulfur acceptor back tozone 1. Also in this embodiment, at least a major portion of solids l7separated from the flue gas stream issuing from zone 2 are wasted viameans 23 rather than recycled as previously described. In all otherrespects, the process remains substantially unchanged from thatpreviously described.

Referring now to FIG. 2, there is shown a fluidized bed boiler modulewhich may be used as combustion zone 2 or for both combustion zones 1and 2 of FIG. I. A bed of granular particles 30 is maintained in afluidized suspension by the action of air or other gases from source 31,passing into plenum 32 and through air distributor and bed support means33. Fuel, such as coal in particulate form, is introduced in pneumaticsuspension through entry means 34 and is burned in fluidized suspensionreleasing head and products of combustion including various forms ofsulfur. A particulate sulfur acceptor, such as limestone, delomite orother calcium and magnesium containing compounds is introduced into thebed through entry means 35. Flue gas, carrying fine particles 36 of flyash, sulfated acceptor material and partially burned carbon particlesexits from the module through breeching 37. Also provided are immersedheat exchange surface 38 and viewed heat exchange surface 39.

Operating temperature of the fluidized bed boiler is maintained at thechosen level by adjusting air and fuel rates; by raising or lowering theheight of the fluidized bed; by changing the amount of immersed andviewed heat exchange surface provided; by changing the ratio of immersedto viewed heat exchange surface or by combinations of the above. Ratioof fuel to air is maintained relatively constant so as to maintain fromabout 0.5 to about percent unreacted oxygen in the produts ofcombustion. Generally we prefer to control operating temperature of theboiler by adjusting the amount of immersed heat exchange surfaceprovided.

The module illustrated can serve as the combustor for either zone 1 orzone 2 of the process of FIG. 1. When operated in a combustion-sulfuraccepting mode, as in zone 1, temperature of the fluidized bed ismaintained within a range of about l,500 to l,700 F. When operated in acombustion-decomposition mode less immersed heat exchange surface isprovided and the bed temperature is maintained at a higher level; withinthe range of l,700 to 2,200 F.

FIG. 3 shows a preferred embodiment of our invention in which both thefirst and second combustion zones comprise fluidized bed combustormodules of the type illustrated in FIG. 2. A plurality of modules 50comprise theflrst combustion zone in which a sulfur containing fuel isburned in conjunction with a sulfur acceptor at temperature betweenabout l,500 t0 l,700 F. Module 51 comprises the second combustion ofdecomposition zone in which temperatures above about l,700 F aremaintained.

Semi-isolated fluidized beds 52 comprising or containing a solidparticulate sulfur acceptor are maintained within both the first andsecond combustion zones. The beds are supported upon air distributiongrids 53. Air, which supplies oxygen for combustion and maintains theparticulate beds 52 in a fluidized condition, is supplied by fan means54 and is distributed among the beds by plenum chamber 55.

The semi-isolated condition of the fluidized beds is maintained by heatexchange wall members 56, S7, 58 and 59. Exterior wall members 56 and 59comprise a conventional water wall while interior wall members 57 and 58are in the form of tubes and connecting fins extending from steam drum60 to headers 61. Details of members 57 and 58 are shown in FIG. 4.Additional viewed or radiant heat exchange surface 62 may be providedwithin each module above the fluidized bed.

In operation, a sulfur-containing fuel such as coal is introduced intothe first combustion zone modules 50 by pneumatic injection through feedports 63. A sulfur acceptor, preferably particulate limestone, may beintroduced in admixture with the fuel. Gaseous products of combustioncarrying fly ash and small particles of sulfated acceptor material arecarried out of the first combustion zone via breeching means 64 and arepassed to solid separation means 65 which may comprise a cyclone.Separated particulate matter consisting of ash, limestone derived matterand unburned solid combustible is passed from separator 65 to the secondcombustion zone module 5 by way of conveying means 66 and injectionconduit 67. Module 51 is operated in the oxidizing mode and ismaintained at a higher temperature than modules 50. Module 51 performsthe function of the fly ash burn-up cell of U. S. Pat. No. 3,508,506 andalso decomposes sulfated acceptor material releasing sulfur oxides andregenerating the acceptor.

It is preferred that a sulfur acceptor, such as a relatively attritionresistant limestone comprise the particulate material making up thefluidized bed. A size in the range of 8 +40 mesh will in most cases besatisfactory. Means are provided (shown in FIG. 4) to allow circulationof bed material from one module to another and from the first to thesecond combustion zones. When using coal as a feed, a size consist ofabout one-fourth inch X 0 is appropriate. This can result in a build-upof ash inventory within the fluidized beds. Means 78 are provided indecomposition module 51 to withdraw, either continously orintermittently, a portion of bed material. Withdrawn bed material may bedisposed of as waste or a substantial portion may be reinjected intocombustion zones 50 since it will contain a high concentration ofregenerated acceptor material. When bed material is recycled, there isproduced a positive flow of sulfated acceptor from the first to thesecond combustion zones and a resupply of regenerated sulfur acceptor tothe first zone.

Auxiliary fuel from supply means 68 is metered into injection conduit 67via conduit and feed means 69. An air stream 70 from any convenientsource pneumatically transports fuel and particulate material throughconduit 67 for injection into the fluidized bed of module-51. Gaseousproducts of combustion and decomposition, maintained separate from thecombustion products of module 50, are passed bia breeching means 71 tosolids separator 72. Particulate solids comprising fly ash and at leastsome finely divided regenerated acceptor material are removed fromseparator 72 by way of line 73. These solids may be discarded or, inthose cases where large concentrations of regenerated acceptor areincluded, may be recycled back to modules 50.

A solids-depleted gas stream is passed from separator 72 to sulfur oxideremoval means 74 by way of conduit 75. Means 74 may comprise anyconventional technique of sulfur oxide removal and/or recovery. It ispreferred that-means 74 comprise a process and apparatus to recovereither elemental sulfur or sulfuric acid from the sulfur oxidescontained in the,gas stream. For example, a sulfur oxide stripping andreaction technique such as that disclosed in U. S. Pat. No. 3,441,379may be'used to recover elemental sulfur while a conventional contactprocess plant may be used to produce sulfuric acid. Alternatively, means74 may comprise a sulfur oxide extraction process which does not recovermarketableforms of sulfur such as scrubbing with lime water.

Since the gas stream from zone-51 is of much less volume, typicallycomprising about 10 percent of the total flue gas stream, and is much'higher in sulfur oxides, typically enriched bya factor of about 10, thanis a conventional flue gas stream, recovery of sulfur or sulfuric acidcan in some cases result in an economic profitsln those cases, it ispreferred that the auxiliary fuel '68 used in module 51 have a highsulfur content. Such fuels are much less costly than are conventionalfuels and produce-higher concentration of sulfur oxides in thecombustiongases. For example, pyritic coal washery refuse and sludgesfrom the sulfuric acid treatment of hydrocarbons may be used toadvantage as the auxiliary fuel in module 5].

In many cases, extraction of sulfur oxides from a waste gas streamrequires substantial cooling; often to 1 a temperature in the range ofabout 90 to 170 F.

Cooling is usually accomplished by water-scrubbing which results insaturating the gas stream with water vapor. When such a cooled,saturated gas stream is released to the atmosphere, it tends to create avery large, low level steam plume. In order to overcome the lack ofbuoyancy inherent in a coolv gas stream and to avoid creation of a steamplume, it is preferred to merge the sulfur oxide depleted gas stream 77from treating means 74 with the larger volume, hotter gas stream 76 fromseparator 65 prior to passing the combined stream to a stack.

Referring now to FIG. 4, there is shown construction details of interiorheat exchange walls which act to maintain the fluidized beds of theseparate modules in a semi-isolated condition. The heat exchange wallsare made up of a plurality of heat exchange tubes 80 terminating attheir lower ends in a manifold or header 61.

Air distribution grid 53 is supported at a level somewhat above header61. Extending between heat exchange tubes 80 are fin members 81. In allcases, the lower edge of their-fin members terminate at alevelintermediatein the expanded height of the fluidized'bed and at alevel above gas distribution grid 53. This construction results in theprovision-of a number of ports or orifices 82 communicatingbetweenadjacent fluidized beds. ln the case of interior heat exchange wellsseparating modules of the first combustion zone, fin members 81terminate at their upper edge at a level approximating the height of theexpanded fluidized bed within the module. Hence, in this instance, thereis free communication of combustion gases above the fluidized bedsbetween the various modules making up the first combustion zone.

The interior heat exchange wall separating the first and secondcombustion zones is similar in construction to that previously describedexcept for one major difference. In this case, fin members 81 extend allthe way to the top of the gas space above the fluidized beds thuspreventing gas mixing with the combustion products of the first zone.This type of construction allows circulation of particulate bed materialbetween modules and between combustion zones ln a preferred mode ofoperation, particulate sulfur acceptor material such as limestone ismaintained as the major constituent of the fluidized bed. As sulfur isreleased by burning of fuel in the first combustion zone, it reacts withthe acceptor material making up the bed. Lateral movement and migrationof the bed material from the first to the second combustion zone resultsin the capture of sulfur in the first zone and release of sulfur in thesecond zone. An auxiliary means to provide additional circulationbetween the two zones may also be provided as has been previouslydescribed.

As may now be appreciated, we have provided a simple but effectivetechnique and apparatus for concentrating a major portion of sulfurcompounds within a minor portion of flue gas. Whilewe prefer to uselimestone as the sulfur accepting material, other calcium or magnesiumoxide-containing natural'materials or waste products may be utilized asis well recognized in the art of sulfur removal. Although thisdisclosure is oriented primarily toward the generation of steam usingcoal as a fuel, the usefulness of our invention is much broader. Forexample, our invention may be used for the incineration of many oilrefinery and chemical plant wastes which contain sulfur with theconcomitant generation of stream or other vapor.

What is claimed is:

1. Combustion apparatus for the burning of sulfur containing fuels andthe extraction of heat energy therefrom which comprises:

a first fluidized bed boiler having heat exchange surfaces disposedwithin the fluidized bed;

a second fluidized bed boiler positioned adjacent said first boiler andhaving less heat exchange surface disposed within the fluidized bed thansaid first boiler;

means to provide circulation of particulate material making up thefluidized beds between said first and said second fluidized bed boilers,and

means to prevent mixing of gaseous combustion products produced in saidfirst boiler with combustion products produced in said second boiler.

2. The apparatus of claim 1 wherein said second fluidized bed boiler ispositioned continguous to said first said boilers.

a location below the top of the fluidized beds.

5. The apparatus of claim 4, wherein said ports are at a level in thelower half of the fluidized beds.

, UNE'EED STATES PATENT OFFICE CER'WWCAIE 0%- CGRECTIGN Patent No. 3,765585 6 Dated October 9, 1975 Inventofls) Ernest B. Robison; SheltonEhrlioh,mJohr1 w. Bishop It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Item ['75] Ernest Be. Robinson" should read I Ernest B, Robison Signedandsealed this 1st day of October 197A.

(SEAL) Attest:

McCOY M. GIBSON JR. Cu MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 60376-1 69 u s GOViRNMENT PRINTING OFFICE: 930

FORM PO-105O (10-69)

2. The apparatus of claim 1 wherein said second fluidized bed boiler ispositioned continguous to said first fluidized bed boiler.
 3. Theapparatus of claim 2 wherein said means to prevent mixing of gaseouscombustion products produced in the two boilers comprises a heatexchange wall member extending from a point below the top level of thefluidized beds continuously to the top of said boilers.
 4. The apparatusof claim 3 wherein said means to provide circulation of particulatematerial making up the fluidized beds between said first and secondboilers comprise ports in said heat exchange wall member at a locationbelow the top of the fluidized beds.
 5. The apparatus of claim 4,wherein said ports are at a level in the lower hAlf of the fluidizedbeds.